Radio navigation system

ABSTRACT

An improved iso-phase line-of-position navigation system for determining the position of a mobile station is disclosed which includes first and second transmitting base stations at respectively fixed locations and a third transmitting station positioned at a movable location. A mobile station is disclosed which includes a receiver for receiving signals from the two base stations and from the third station and means responsive to the signals received at the receiver for determining the position of the mobile station along first and second iso-phase lines-ofposition relative to the first and second base stations and the third station. Computing means are provided which are responsive to the position of the mobile station relative to the first and second base stations and the third station and also to the absolute location of the third station for computing the position of the mobile station.

United States Patent Hastings et al.

[ Jan. 22, 1974 I RADIO NAVIGATION SYSTEM Primary ExaininerMaynard R.Wilbur Assistant ExaminerRichard E. BIer er 751 t:ChlE.Ht,N rtN; g l 1men Ors ngs {ar a igf Attorney, Agent, or F irm- Ronald W. Reagln andComstock, Newport News; Joseph Ralph Braunstem '\l/.aI3radbury, VirginiaBeach, all of ABSTRACT An improved iso-phase line-of-position navigationsys I73] Asslgnee: Teledynei Inc-I L05 Angeles, tern for determining theposition of a mobile station is [22] Filed; May 8, 1972 disclosed whichincludes first and second transmitting base stations at respectivelyfixed locations and a third PP 251,470 transmitting station positionedat a movable location. A mobile station is disclosed which includes areceiver [52 US. Cl. 343/6 R, 343/12 R, 343/15, for receiving Signalsfrom the two base Stations and 343 05 R, 343/112 R from the thirdstation and means responsive to the sig- [5]] Int. Cl G015 9/02 receivedat the receiver for determining the posi' 53 Field f g 343/112 11 12 R,105 R, 15 tion of the mobile station along first and second isophaselines-of-position relative to the first and second i 5 References Citedbase stations and the third station. Computing means UNTED STATESPATENTS are provided which are responsive to the position of 2 724 1 l411 955 K f 343 105 R the mobile station relative to the first and secondbase 3'242494 3x966 35 34341 12 R stations and the third station andalso to the absolute 34O0399 9,1968 Kline 343/112 R location of thethird station for computing the position of the mobile Station.

7 Claims, 4 Drawing Figures THIRD STATION "I I8 I W I I TRANSMITTER I l22 RECEIVER I l I I POSITION 30% TRANSMITTER I MOBILE sTATIoN POSITION/34 W7 COMPUTER A I 24/1 32 I i 26 I 28" 1 I THIRD SECOND I RECEIVERRECEIVER l I I l u g fl w w J FIRST BASE STATION SECOND BASE STATIONPAIENTEDJANZZIQH a; TBT. 844

' FIRST SECOND IO BASE BASE I2 STATION STATION THIRD STATION P I V 11msl/- CW I TRANSMITTER I I f 22 RECEIVER I I I I I POSITION I DOCI/TRANSMITTER MOBILE STATION IS F h T I I POSITION /34 7 I COMPUTER I 24432 I L 26 I 28 1 I I THIRD SECOND I I RECEIvER RECEIvER I I l t I FIRSTSECOND |O-/ BASE BASE I2 STATION STATION PATENTED- 3.787. 844

SHEET 3 0F 22 THIRD STATION I CW I I8-: TRANSMITTER i l I I 22 20REcEIvER 16 i J, i FIRST I 36? RECORDER I L J MOBILE STATION 24 T T TSECOND I K REOEIvER I 28 SECOND I 38 RECORDER I L J FIRST SECOND -IO-BASE BASE I2 STATION STATION iflig. 4

POSITION J34 OOIvIRuTER FIRST SECOND J REOOROER REOOROER RADIONAVIGATION SYSTEM This invention relates to radio navigation systems,and more particularly to an improved non-saturating iso-phaseline-of-position navigation system which can be used in conjunction withexisting navigation systems such as range-range iso-phaseline-of-position navigation systems.

There are several types of iso-phase line-of-position navigation systemswhich are presently in widespread use to determine the precise positionof a ship or other vessel which is navigating in the general vicinity(within 200 to 300 miles) of a shoreline. Two such systems known tothose skilled in the art are the circular line-ofposition system, alsoknown as the range-range system, and the hyperbolic line-of positionsystem. In each of these systems, transmitters or base stations areprovided along the shoreline at known fixed locations and signalsbroadcast by these base stations are received by a vessel. The vesselcontains suitable means for determining the phases of the signalsreceived from the base stations and from the phases of these signalsdetermines the position of the vessel along an iso-phase line-ofpositionrelative to each of the base stations. The intersection of two suchlines-of-position provide a fix or definite location of the vessel atthat time.

However, there are certain drawbacks associated with each of these typesof iso-phase navigation systems. ln the case of the circular orrange-range system, the vessel itself must also transmit a signal to thebase stations. Thus, because of frequency allocations and because of thedifficulty in separating signals of very close frequency which might bebroadcast from different vessels in the same vicinity, there aredefinite limits on the number of vessels or ships which can operate inthe same area using the same base stations. At the present time, thepractical limit of the number of vessels which can use the same basestations at the same time is only three or four vessels. Thus, such asystem is termed a saturating system, because of the limitation in thenumber of vessels which can use the system at the same time. Anotherproblem with the circular or rangerange system is that'the accuracy ofthe system diminishes substantially as a vessel approaches the shorelinebetween the two base stations. This degraduation in performance in thisarea is caused by the sharp angles of intersection of the iso-phaselines-of-position along the straight or base line between the two basestations. Thus, while the range-range system is noted for its extremeaccuracy of position, at least when the vessel using the system is asufficient distance away from the shoreline, it is also known that suchsystems suffer from the two defects described above.

The hyperbolic line-of-position system overcomes some of the defects ofthe range-range system described abive, but suffers from separatedefects of its own. The hyperbolic system requires at least threetransmitters which are not located in a straight line with each other.Thus, if the shoreline along which the base stations are to be locatedis a generally straight shoreline, the center station must be positionedsome distance inland from the shoreline. This leads to seriousperformance problems, since it is known that the range of a signal beingtransmitted at a given power level is much greater over water than it isover land. Experience has shown that in many instances, over poorterrain, a signal of a given strength will travel only about 10 milesover land, whereas the same signal could travel 200 to 300 miles overwater. Thus, if the central station must be located up to 10 milesinland from the shoreline in an area of poor terrain, such as overjungle, mountains or the like, vessels operating offshore frequentlyreceive no signals whatsoever from this station, and the entire systemis rendered inoperative.

It is also known to those skilled in the art that the effective areawhich can be covered by a hyperbolic lineof-position system isconsiderably less than the area which can be covered by a range-rangesystem. since the area over which the angles ofintersection of thehyperbolic lines-of-position are close to rectilinear intersections,which is necessary for high accuracy. is considerably less than the areaover which the circular lines of position so intersect.

However, the hyperbolic line-of-position system is a completelynon-saturating system, in that the vessel navigating in the system needcarry only a receiver, and does not itself broadcast any signals to thebase stations. Thus, an unlimited number of vessels may use the systemsimultaneously on the same frequency bands.

One approach which has been used with some degree of success in theprior art is to provide a hyperbolic line-of position system in whichtwo base stations are positioned along the shore-line, in a mannersimilar to conventional hyperbolic systems and to range-range systems,and the third base station is positioned somewhere offshore, such as onan island, if one is conveniently located, or on some other fixedplatform such as an offshore oil well drilling platform. It has beenfound that very good accuracy can be obtained by using a hyperbolicsystem on a vessel which is operating generally within the trianglebounded by the two base stations and the offshore station, since in thisarea the angles of intersection of the lines-of-position are extremelygood. However, the use of this type of system has obviously been limitedto those few areas which include a suitable offshore location forlocating the third base station.

It is accordingly an object of the present invention to provide animproved iso-phase line-of-position radio navigation system.

It is yet another object of the present inventionto provide an improvediso-phase line-of-position radio navigation system which requires onlytwo base stations positioned along the shoreline, and thus requires nobase station position some distance inland from the shoreline.

It is yet another object of the present invention to provide an improvedsuch radio navigation system which is non-saturating, that is which canbe used simultaneously by a large number of mobile stations to determinetheir respective positions.

Briefly stated, and in accordance with the presently preferredembodiment of the invention, an improved iso-phase line-of-positionnavigation system for determining the position of a mobile station isprovided which includes first and second base stations at respectivelyfixed locations located along the shore-line and a third stationpositioned at a movable location at sea which includes a transmitter forbroadcasting signals at a predetermined frequency. Conveniently, thefirst and second base stations and the third station can themselves bethe conventional components of a rangerange navigation system of thetype described above in which the third. station is carried on a vesseland the range-range system is used in the conventional manner todetermine the position of that vessel relative to the two base stations.In accordance with the present invention; a mobile station whoseposition is to be determined is provided which includes a receiver forreceiving signals from the two base stations and from the third stationand means responsive to the signals received at the receiver fordetermining the position of themobile station along first and secondiso-phase lines-of position relative to the first base station and thethirdstation and the second base station and the third stationrespectively. Conveniently, the mobile station can be carried by alaunch which is operating in conjunction with the above mentioned vesselcarrying the third station. Computing means are provided which areresponsive to the position of the mobile station relative to the firstand second base stations and the third station and also to the absolutelocation of the third station for computing the position of the mobilestation independently of the position of the third station.

For a complete understanding of the invention, and an appreciation ofits other objects and advantages, please refer to the following detaileddescription of the attached drawings, in which:

FIG. 1 is a block diagram of a range-range iso-phase line-of-positionnavigation system such as is known in the prior art;

FIG. 2 is a block diagram of an iso-phase line-ofposition navigationsystem in accordance with the presently preferred embodiment of thisinvention;

FIG. 3 is a block diagram of a iso-phase line-ofposition navigationsystem in accordance with a second embodiment of the present invention;and

FIG. 4 is a block diagram showing the manner in which the system of FIG.3 determines the absolute position of the mobile station.

FIG. 1 shows a block diagram of a typical rangerange iso-phaseline-of-position navigation system such as is well known to thoseskilled in the art. This prior artsystem is shown and described here toprovide a proper antecedent for the description of the system of thepresent invention below and to show the manner in which the presentinvention utilizes presently available equipment to overcome the defectsin the prior art system noted above.

vBriefly, the range-range system of FIG. l'includes a first base stationand a second base station 12 which are typically positioned along ashoreline 14. The system also includes a third station 16 whose positionis to be determined. Typically, the third station 16 is carried aboard avessel or the like which is navigating on the sea in the generalvicinity of the base stations 10 and 12. Range-range systems presentlyavailable are operational for distances up to 2 to 300 miles from thebase stations 10 and 12, so the vessel carrying the third station 16 canbe navigating anywhere within this region.

F such that a multiple of the carrier signal differs from the frequencyF,, by an amount Af which is itself usually a frequency in the audiorange. The value of Af for each of the base stations 10 and 12 issufficiently different that signals of these frequencies can easily besegregated for separate handling in the receiver 20.

Thus, using the terminology and designation just given, the frequency ofthe carrier wave being broadcast by the first base station 10 can bedesignated (F,, Af )/2 and the frequency of the carrier wave beingbroadcast by the second base station 12 can be designated (F,,,+Af2)/2.

The transmitter in the first base station 10 includes a local oscillatorwhich is free running at the frequency (F,,,Af )/2. No attempt is madeto regulate the phase of this oscillator relative to the phase of thesignal being broadcast by the CW transmitter 18 of the third station 16.Instead, the local oscillator signal at the first base station 10 isdoubled in a frequency multiplier at base station 10 and heterodynedwith the signal F received at base station 10 to develop a firstmodulation signal whose frequency is Af This first modulation signal ismodulated onto the carrier wave at first base station 10. Any desiredform of modulation, such as amplitude modulation or frequency modulationmay be used, but in the preferred system available today, the modulationsignal is modulated as a single sideband onto the carrier signal. Thissignal sideband modulation may be either an upper sideband or a lowersideband modulation, but to provide optimum frequency separation, it ispreferred that whichever of the two base stations 10 and 12 is operatingat the lower frequency use a lower single sideband modulation and theother base station use an upper single sideband modulation.

The modulated signal as broadcast by first base station 10 is nowreceived by the receiver 20 in the third station 16. The receiver 20separates the modulation signal from the carrier wave and also placesthe carrier wave in a frequency multiplier to double its frequency. Thedoubled carrier wave is then heterodyned with the signal from the CWtransmitter 18 to develop an additional signal which has the frequencyAf This signal and the modulation signal as demodulated by the receiver20 are then placed in a phase meter to determine the difference in thephases of these two signals of frequency Af,. As is well known to thoseskilled in the art,

The third station 16 includes a continuous wave 1 this difference inphase is the indication of the range between the third station 16 andthe first base station 10, and places the third station 16 in a circularisophase line-of-position around the first base station 10.

In a similar manner, the second base station 12 receives the signal offrequency F,, from the third station 16 and transmits a modulated signalhaving a carrier frequency (F,,,+Af )/2 and having a second modulationfrequency of signal Af modulated thereon, which signal is received bythe receiver 20. The receiver 20 demodulates this signal to recover thesecond modulation signal and also doubles this carrier signal andheterodynes it against the signal from the CW transmitter 18 to developa second signal also having the frequency Af Again, the difference inthe phases of these two signals is an indication of the range betweenthe third station 16 and the second base station 12, and places thethird station 16 along a circular iso-phase line-ofposition around thesecond base station 12. The intersection of these two circularlines-of-position so determined establishes a definite location or fixof the position of the third station 16.

In a typical range-range'navigation system of the type just described,the frequency F of the CW transmitter 18 might be 3300.400 kI-Iz, thefrequency Af might be 370 I-Izand the frequency Af might be 450 Hz.Thus, the carrier frequency of the first base station would be 1650.015kHz and the carrier frequency of the second base station 12 would be1650.425 kHz.

No internal details of the base stations 10 and 12 or the receiver areshown in the description given above, since such components per se arewell known to those skilled in the art, and in themselves form no partof the present invention. For example, details of such base stations areshown in the co-pending U.S. Pat. application Ser. No. 96,767, filedDec. 10, 1970 and assigned to the assignee of the present application.

FIG. 2 shows a block diagram of an iso-phase line-ofposition navigationsystem in accordance with the present invention for determining theposition of a mobile station 24 which might be navigating in the generalarea bounded by the first and second base stations 10 and 12respectively, and the third station 16 of the range-range system of FIG.1 described above. For example, the mobile station 24 might be carriedon a launch or the like which is operating in conjunction with thevessel carrying the third station 16. Thus, the vessel might be termedthe mother vessel of the launch carrying the mobile station 24.

The mobile station 24 includes a second receiver 26 which receives itsinput signals through an antenna 28. The second receiver 26 can begenerally similar to the receiver 20 in the third station 16. In themanner well known to those skilled in the art, the second receiver 26receives the signals being broadcast by first and second base stations10 and 12 respectively and third station 16 and determines the positionof mobile station 24 along two sets of iso-phase lines-of-positionrelative to these three stations. Thus, the intersection of these twolines-of-position determines the position of the mobile station 24relative to these three stations. For example, second receiver 26 canreceive the signal F being broadcast by the third station 16 and the twocarrier signals and modulation signals being broadcast by the basestations 10 and 12. In the manner described in the above mentionedco-pending application Ser. No. 96,767, filed Dec. 10, 1970, this issufficient information for second receiver 26 to determine the positionof mobile station 24 relative to these three stations, either alonghyperbolic iso-phase linesof-position having the base line first basestation IO-third station 16 and second base station l2-third station 16or along halops isophase lines-of-position. Since the manner in whichsecond receiver 26 operates is not part of the present invention, andsince its manner of operation is well known to those skilled in the art,no further description of the structure or operation of this receiver isgiven herein.

However, it will be appreciated that, while second receiver 26 gives theposition of mobile station 24 relative to first and second base stations10 and 12 and third station 16, this does not determine the absoluteposition of mobile station 24, since the third station 16 is beingcarried on another vessel which itself might be moving about the sametime as is the launch carrying the mobile station 24. In accordance withthe present invention, this is compensated for by providing a positiontransmitter 30 on third station 16 which receives its input signal fromthe receiver 2 0 which indicates the position of the third station 16 atthe particular instant. This position indicating signal is thentransmitted by position transmitter 30, conveniently through antenna 22,and is received at the mobile station 24 on antenna 28 and applied to athird receiver 32, which can be a conventional receiver which iscompatible with the format of the signal being broadcast by positiontransmitter 30. Thus, the output of the third receiver 32 indicates theinstantaneous position of the third station 16. The output of thisreceiver and the second receiver 26 are applied to a position computer34 which computes the instantaneous absolute position of mobile station24 independent of the position of the third station 16. This is arelatively simple computing task, since the locations of first andsecond base stations 10 and 12 are known and fixed, and the position ofthird station 16 is known through the third receiver 32. Since all threeof these positions are now known to position computer 34, and since thesecond receiver 26 gives the position of mobile station 24 relative tothese three stations, the position computer 34 can easily compute theabsolute position of mobile station 24 at that time.

It will be appreciated by those skilled in the art that, since mobilestation 24 requires no transmitter, the system just described is anon-saturating system, and any number of mobile stations carried onseparate launches or the like can use a single range-range system inwhich the third station 16 is carried on a mother vessel or the like todetermine the position of the mobile stations 24. It is a non-saturatingsystem which is not subject to the limitation on the number of users:such as is present in conventional range-range systems. Further, itdoes not require a third station located further inland on the shore,such as would be required by a conventional hyperbolic iso-phaseline-of-position navigation system. The mobile station 24 can alsooperate right up to the shoreline 14 with high positional accuracy, andwithout being subject to performance degradation such as occurs inconventional range-range systems when the vessel moves close to theshoreline.

Position transmitter 30 and third receiver 32 can take any desired formfor transmitting the positional information of third station 16 to themobile station 24. In fact, in instances where it is feasible to do so,this information need not even be broadcast, but can be transmitted overcables and wires connecting the third station 16 and the mobile station24. Alternatively, the position transmitter 30 can be a conventionaltransmitter which transmits the position of third station 16 in digitalformat and the third receiver 32 can be a corresponding receiver toreceive and demodulate this information and to apply it to the positioncomputer 34. Such an arrangement provides an operating system which isindependent of the distance between the mobile station 24 and any one ofthe three stations first base station 10, second base station. 12 andthird station 16.

However, in the presently preferred embodiment of the invention, it hasbeen found that a very simple, effective and inexpensive means fortransmitting the desired positional information is to merely relay thefirst and second modulation signals recovered in the receiver .20 fromthe first base station 10 and the second base station '12 respectivelyto the mobile station 24. In this embodiment, the position transmitter30 merely becomes a simple modulator which modulates these first andsecond modulation signals, or frequency multiples of these signals, ontothe signal of frequency F being transmitted by the CW transmitter 18.

Now in this preferred embodiment, the third receiver 32 is again aconventional iso-phase line-of-position receiver like the receiver 20 inthird station 16 and the second receiver 26. This third receiver 32 now'receives the first modulation signal as relayed by the positiontransmitter 30 and also receives the first modulation signal directlyfrom the first base station 10. The phases of these two modulationsignals, both having the frequency Af are then subtracted.

Computation of this phase difference shows that it is equal to.

( m/ 1) ft/ 1 3 DI) where D is the distance between the third station 16and the first base station 10, d is the distance between the mobilestation 24 and the first base station 10, 41;, is the distance betweenthe mobile station 24 and the third station 16 and c is the propagationconstant, or phase shift per unit distance per hertz. This equationshows that the phase meter showing the difference in the phases betweenthe two signals of frequency Af reproduces the range D between the thirdstation 16 and the first base station 10, provided the correction(Af,/c)(d d D is made. This correction is quite small. since as wasnoted above, the value of P is approximately ten thousand times greaterthan the value Af and could be neglected for most applications. Further,if the launch carrying the mobile station 24 is operated near the motorvessel carrying the third station 16, the valve of the d, approaches thevalue of D, and the value of d approaches zero, and the correctionfactor itself approaches zero.

Similarly, the third receiver 32 also receives the second modulatingsignal of frequency Af directly from the second base station 12 and alsorelayed from the third station 16. Again, the phases of these twosignalsare subtracted and the resultant phase difference is a measure of therange between the second base station 12 and the third station 16, againassuming the above described correction is made.

' The positional information of the third receiver 32 is then fed intothe position computer 34 to obtain the absolute position of the mobilestation 24 at that instant.

It is sometimes not important or necessary that an operatoron the launchcarrying the mobile station 24 know precisely where he is at any giveninstant. Instead, it is only necessary that the position of the mobilestation 24 at a given time can be subsequently calculated. For example,if the mobile station 24 is being carried on a launch working inassociation with a hydrographic or geophysical survey vessel carryingthe third station 16, and is making measurements or taking samples inconnection with hydrographics or geophysical studies and surveys, it issufficient if the time of the samples and measurements is recorded andthe position of the mobile station 24 at this time can subsequently becalculated.

FIGS. 3 and 4 show a block diagram of a modification of the system ofFIG. 2 which meets the requirements just described but which can beoperated in a simple and more economical manner than the system of FIG.2. In the system shown in FIG. 3, the output of the receiver 20, andthus the instantaneous position of the third station 16, is recorded asa function of time on a first recorder 36 which is carried by the thirdstation 16. Similarly, the output of the second receiver 26, which isthe position of the mobile station 24 relative to the first and secondbase stations 10 and 12 and the third station 16 is recorded as afunction of time on a second recorder 38. Subsequently, as is shown inFIG. 4, the outputs of the first and second recorders 36 and 38respectively are played synchronously into the position computer 34which determines the position of the mobile station 24 at the time thesecond recorder 38 is recording the output signals from second receiver26.

The advantage of the system just described is that only a singleposition computer 34 is required rather than a separate computer foreach of the mobile stations 24. The position computer 34 can now becarried on the mother vessel carrying the third station 16. Thepositional computation can probably even be done on computers which thevessel is carrying anyhow, so that it is not necessary to provideseparate position computers at all. Further, the system requires only asingle receiver on each mobile station, rather than two separatereceivers as does the system of FIG. 2. Thus, economy in hardware can beeffected in those applications where such a system can be used.

While the invention is thus disclosed and several specific embodimentsdescribed in detail, it is not intended that the invention be limited tothese shown embodiments. Instead, many modifications will occur to thoseskilled in the art which lie within the spirit and scope of theinvention. It is intended that the invention be limited inscope only bythe appended claims.

What is claimed is:

1. An iso-phase line-of-position navigation system for determining theposition of a mobile station, comprismg: I

first and second base stations at respective fixed locations eachincluding a transmitter for broadcasting signals at respectivepredetermined frequencies,

a third station positioned at a movable location which includes atransmitter for broadcasting signals at a predetermined frequency, afirst receiver for receiving said signals broadcast by said basestations and means responsive to said first receiver for determining theposition of said third station relative to said base stations,

a mobile station whose position is to be determined, said mobile stationcomprising a second receiver for receiving said signals broadcast bysaid base stations and said third station and means responsive to saidsecond receiver for determining the position of said mobile stationalong a first iso-phase line-ofposition relative to said first basestation and said third station and along a second iso-phaseline-ofposition relative to said second base station and said thirdstation, and computing means responsive to said first and secondreceivers for computing the position of said mobile station. 2. Thenavigation system of claim 1 in which said third station is positionedon a vessel and in which said first and second base stations and saidthird station comprises a range-range iso-phase line-of-positionnavigation system for determining the position of said vessel relativeto said first and second base stations.

3. The navigation system of claim 2 in which said third station furthercomprises position transmitting means for broadcasting signalsindicative of the position of said vessel, said mobile station furthercomprises a third receiver for receiving said signals indicative of theposition of said vessel and in which said computing means is positionedon said mobile station and receives inputs from said second and thirdreceivers to compute the position of said mobile station.

4. The navigation system of claim 3 in which said range-range navigationsystem includes means for modulating the signals broadcast by each ofsaid base stations with a respective modulation signal whose phase is afunction of the range from said base station to said third station, saidposition transmitting means in said third station comprising means fortransmitting said modulation signals as received at said third stationand said third receiver comprises means for receiving said modulationsignals from said base stations and from said third station and forcomparing the phase of each of said modulation signals as received fromsaid third station with the phase of the corresponding modulation signalas received from its respective base station, thereby indicating at saidmobile station the range from said respective base station to said thirdstation.

5. The navigation system of claim 1 which further comprises firstrecording means responsive to said first receiver for recording theposition of said third station as a function of time, second recordingmeans responsive to said second receiver for recording the position ofsaid mobile station relative to said base stations and said thirdstation as a function of time, and means for synchronously applyingoutput signals from said first and second recording means to saidcomputing means to provide the position of said mobile station as afunction of time.

6. The navigation system of claim 4 in which said means responsive tosaid second receiver determines the position of said mobile stationrelative to said base stations and said third station along hyperboliciso' phase lines-of-position.

7. The navigation system of claim 4 in which said means responsive tosaid second receiver determines the position of said mobile stationrelative to said base stations and said third station along a halopsiso-phase line-of-position.

1. An iso-phase line-of-position navigation system for determining theposition of a mobile station, comprising: first and second base stationsat respective fixed locations each including a transmitter forbroadcasting signals at respective predetermined frequencies, a thirdstation positioned at a movable location which includes a transmitterfor broadcasting signals at a predetermined frequency, a first receiverfor receiving said signals broadcast by said base stations and meansresponsive to said first receiver for determining the position of saidthird station relative to said base stations, a mobile station whoseposition is to be determined, said mobile station comprising a secondreceiver for receiving said signals broadcast by said base stations andsaid third station and means responsive to said second receiver fordetermining the position of said mobile station along a first iso-phaselineof-position relative to said first base station and said thirdstation and along a second iso-phase line-of-position relative to saidsecond base station and said third station, and computing meansresponsive to said first and second receivers for computing the positionof said mobile station.
 2. The navigation system of claim 1 in whichsaid third station is positioned on a vessel and in which said first andsecond base stations and said third station comprises a range-rangeiso-phase line-of-position navigation system for determining theposition of said vessel relative to said first and second base stations.3. The navigation system of claim 2 in which said third station furthercomprises position transmitting means for broadcasting signalsindicative of the position of said vessel, said mobile station furthercomprises a third receiver for receiving said signals indicative of theposition of said vessel and in which said computing means is positionedon said mobile station and receives inputs from said second and thirdreceivers to compute the position of said mobile station.
 4. Thenavigation system of claim 3 in which said range-range navigation systemincludes means for modulating the signals broadcast by each of said basestations with a respective modulation signal whose phase is a functionof the range from said base station to said third station, said positiontransmitting means in said third station comprising means fortransmitting said modulation signals as received at said third stationand said third receiver comprises means for receiving said modulationsignals from said base stations and from said third station and forcomparing the phase of each of said modulation signals as received fromsaid third station with the phase of the corresponding modulation signalas received from its respective base station, thereby indicating at saidmobile station the range from said respective base station to said thirdstation.
 5. The navigation system of claim 1 which further comprisesfirst recording means responsive to said first receiver for recordingthe position of said third station as a function of time, secondrecording means responsive to said second receiver for recording theposition of said mobile station relative to said base stations and saidthird station as a function of time, and means for synchronouslyapplying output signals from said first and second recording means tosaid computing means to provide the positiOn of said mobile station as afunction of time.
 6. The navigation system of claim 4 in which saidmeans responsive to said second receiver determines the position of saidmobile station relative to said base stations and said third stationalong hyperbolic iso-phase lines-of-position.
 7. The navigation systemof claim 4 in which said means responsive to said second receiverdetermines the position of said mobile station relative to said basestations and said third station along a halops iso-phaseline-of-position.